A dissertation submitted to the faculty of Engineering, University of the Witwatersrand, Johanuesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering. / Ferritic stainless steels possess a number of properties which render them superior to austenitic stainless steels in certain applications. The ferritic stainless steels are highly resistant to stress-corrosion cracking and are generally of lower cost. Previous work had shown that the corrosion properties of the ferritic steels were optimised at a chromium content of forty percent. Extensive research had already characterised the mechanical and corrosion behaviour of the Fe"40Cr alloy in the cast and wrought form. This investigation involves the development of the material by powder metallurgy processing. Particular emphasis is placed on a proprietary powder production technique as well as on powder consolidation by Hot Isostatic Pressing. The effect of Ni, Mo, Ru, Nb, Al and Fe additions and well as various techniques of introducing these additions are examined. The unique effect of powder metallurgy manufacture on microstructure and the kinetics of sigma phase formation is highlighted. Corrosion tests in various concentrations of sulphuric acid and an industrial field trial were performed. It was found that combined Ni and Mo contents exceeding four percent resulted in severe embrittlement due to widespread formation of sigma phase during the HIP process. The use of powder metallurgy techniques was also found to enhance the kinetics of sigma phase formation in a particular alley when compared to the as-cast state. Direct additions of Fe powder were successful in inhibiting embrittlement, possibly due to a mechanism of Fe diffusion into Cr-rich regions. This lowered of the Cr content in these regions, thereby reducing the tendency for sigma precipitation. Corrosion tests indicate satisfactory resistance for the Fc-40Cr-2Ni-2Mo alloys in 70 weight percent sulphuric acid at 50°C. Alloys with 0.2 percent Ru additions were found to be especially resistant, due to the role of Ru as a cathodic modifier. The method of mixing and diffusion bonding metallic powders of varying composition was found to be feasible and certain novel combinations of Ni and Fe-rich alloys exhibited satisfactory corrosion resistance. A mathematical diffusion model was found to provide an order of magnitude approximation of the time required for hornogentsation in a certain mixture of powders. / Andrew Chakane 2018

Thesis (M. Sc.(Metallurgy))-University of Pretoria, 2006. / Includes bibliographical references (leaves 62-65). Available on the Internet via the World Wide Web.

Thesis (M.Sc.)(Metallurgy)--University of Pretoria, 2004. / Includes summary. Includes bibliographical references.

Includes reprints of author's related articles. / Bibliography: pages 187-201. / The competition between microfracture and plastic flow has been studied in relation to the thermomechanical processing parameters and minor element chemistry of wrought super-ferritic alloys based on a composition of Fe-40wt% Cr. These alloys have been developed for corrosion-resistant applications, specifically by micro-alloying with platinum group metals to induce cathodic modification, but their use has been hampered by inadequate toughness at ambient temperatures. Brittle cleavage of the alloys is a consequence of the high resistance to plastic flow required to accommodate local stresses, such as those found ahead of a loaded crack. Once initiated, a crack propagates in a brittle manner with minimal ductility. The impact toughness therefore relies on the ability of the alloys to withstand crack initiation. The frequency of the crack initiation events is related to the distribution of secondary phases within the matrix and at the grain boundaries. A direct means of improving the toughness and the ductility is accordingly via annealing cycles and minor alloying additions to control the precipitation of second phases. The ductility is enhanced by raising the mobile dislocation density, and this may be achieved by pre-straining recrystallised material, or increasing the number of dislocation sources in the otherwise source-poor material. The generation of mobile dislocations by prismatic punching at second phase particles in response to local or tessellated stresses was found to increase the ductility and the impact toughness of the alloy. The addition of nickel also increases the brittle fracture resistance by promoting stress accommodation at the crack tip, a result which can, in principle, be explained on the basis of enhanced dislocation dynamics. The tendency of the alloys to form a stable recovered substructure was identified as a critical parameter for both the mechanical and corrosion properties. The low-angle dislocation sub-arrays contribute to overall strain-hardening, but destabilise the passivity of the alloys in acid media. In practice, rationalisation of the microstructural parameters has enabled the practicable fabrication of tough, corrosion-resistant alloys, suitable for commercial development.

Bibliography: pages 107-108. / The effects of exposure to temperatures in the range 475 - 800°C on the hardness and associated microstructure of high chromium ferritic steels has been investigated. Low-carbon 26Cr-1Mo steels containing 0,02 - 0,04% nitrogen were found to constitute an age hardening system when quenched from a temperature of nitrogen solubility and exposed at temperatures in the range 600 - 700°C. TEM observations on thin foils revealed that hardening was associated with the formation of a high density of Cr-N zones. These grew on over-ageing into disc-shaped Cr-N precipitates situated on {100} lattice planes, and ultimately became large incoherent precipitates. Ageing at 475°C and 550°C produced hardening due to the formation of chromium-rich ferrite phases α' as a result of the miscibility gap in the Fe-Cr phase diagram. However the presence of interstitial nitrogen in solution in the steel considerably reduced the rate of hardening, especially at 475°C. TEM examination confirmed that this effect was due to the formation of Cr-N zones in preference to α'. This type of decomposition occurs by a mechanism of nucleation and growth, forming zones similar to those formed during an ageing at 600°C. When depleted of interstitial nitrogen, through precipitation at 800°C or through zone formation at 475°C, the specimens aged at 475°C underwent spinodal decomposition. Thus nitrogen in solid solution was found to have a significant effect on the 475°C hardening reaction. Precision X-ray diffraction measurements revealed the presence of secondary diffraction peaks associated with the Bragg peaks, which comfirmed the formation of Cr-rich phases during ageing at 475°C. The calculated associated lattice parameter measurements allowed estimates of the compositions of the decomposition phases to be made. These were calculated to be about 6-18% Cr in the Fe-rich and 60-80% Cr in the Cr-rich phases of the 26Cr-1Mo steel.

Diffusion couples between nickel and S.A. 212 ferritic steel and between nickel and AISI 304 austenitic stainless steel were studied to determine the effect of nickel on the structure of these steels after diffusion anneals at 1300 ℉. Diffusion times varied from 50 to 4000 hours. The migration of nickel resulted in the formation of a martensitic band between nickel and S.A. 212 ferritic steel and an austenitic band between nickel and AISI 304 austenitic stainless steel. The width of the bands increased exponentially with the time of annealing. The band width increased faster in nickel-S.A. 212 couple than in nickel-AISI 304 couple. Hardness values were obtained within the band of both diffusion couples and varied across the band. Generally, the hardness was greatest in the band. In the nickel-AISI 304 diffusion couple, chromium carbides were observed in the nickel plate after diffusion. / M.S.

The behaviour of mould fluxes used in continuous casting of two Ti-stabilised stainless steels was investigated in terms of the level of titanium pick-up by the flux and the effect of this absorption of titanium on the viscosity of the fluxes. The two fluxes considered are respectively used for the casting of a ferritic steel (type 409) and an austenitic steel (type 321). Concerning the titanium pick-up (expressed as Ti02), the Ti02 content of the flux stabilised at about 3-4% for the mould flux of the ferritic steel and at about 6% for the mould flux of the austenitic steel after 20 minutes of casting. At the same time due to the reduction of Si02 in the molten flux by TiN and Ti in the steel the basicity of the mould flux of the ferritic steel increased from 0.8 to 0.9 while it increased from 0.95 to 1.2 for the mould flux of the austenitic steel. The SEM/EDS analysis of the sampled flux during casting showed only some spherical metallic droplets in the case of the mould flux of the ferritic steel but for the mould flux of the austenitic steel apart from the metallic droplets, some precipitates rich in Ca, Ti and O were identified in the glassy phases. Rotational viscometry carried out on the two fluxes showed that there is a decrease in the viscosity of the fluxes with the absorption of Ti02, Ti2O3 and Ti3O5 in the range of 2 to 10 wt%, for temperatures from 1400°C to 1200°C. The effect of Ti02 and Ti2O3 has been tested with the mould flux of the austenitic steel at a basicity of 1.2 to match the basicity which arises during casting. For temperatures of 12500C and below, the apparent viscosity of the flux increased markedly with the absorption of 10 % of Ti02 or Ti2O3. In both cases precipitation of perovskite (Ca2 Ti2O6 or Ca2 Ti2O5) was found to be responsible for the increase of the apparent viscosity of the flux of the austenitic steel. / Dissertation (M Eng (Metallurgical Engineering))--University of Pretoria, 2006. / Materials Science and Metallurgical Engineering / unrestricted

Please read the abstract in the section, 00front, of this document / Dissertation (MEng)--University of Pretoria, 2008. / Materials Science and Metallurgical Engineering / unrestricted

The aim of this work was to carry out an experiment to assess the influence of the individual welding parameters in the laser welding method with the beam wobbling on the made weld, respectively. welds. The material which was welded was ferritic stainless steel with the designation 1.4016, which, like all ferritic steels, is susceptible to coarse grains and the resulting brittleness which leads to degradation of the mechanical properties of the weld. The welds were compared to a sample welded without wobbling an between themselves. Unfortunately, however, the evaluation of the test samples did not bring significant differences from laser welding without wobbling, however some changes are evident, such as grain orientation, weld defects, or uniformity of mechanical properties over the weld length, where wobbling frequency is most important. Further research should focus on limiting the porosity at higher wobbling frequencies in the shape of a circle or ellipse.

Heterogeneous welded joints – type ferrite/austenite is inseparable part of structure for energy industry. Welding conditions and post weld heat treatment have a significant impact on the structural stability of welded joint. The structure determines resulting mechanical properties that determine lifetime of these joints. At the same time in microstructure changes in heat affected zone of the base material is diffusion of carbon through the weld interface. This work is focused on the analysis of structural stability of heterogeneous weld carbon/austenitic steel, which was formed carbon steel 22K (base material) and austenitic buttering layer EA 395/9 with a higher content of nickel. Evaluation of the structure was focused on the structure of weld metal (Böhler FOX SAS 2) and structure of the heat affected zone of base material. Metallographic evaluation was performed on the light microscope and scanning electron microscope equipped with the energy and wave dispersive analysis, which identified the contents of substitutional elements and carbon on the interface of carbon/austenitic steel. The results of metallographic analysis were compared with measurements of Vickers hardness and microhardness and calculations using Thermo-Calc software and software SVARY. The description and evaluation of the stability of the weld joints was determined by carbon content of the course interface using wave dispersive spectroscopy. The results of energy dispersive analysis were used to determine the type of carbides present at the interface of carbon/austenitic steel.